Broadband biconical wire-grid lens antenna comprising a central beam shaping portion



Feb. 8, 1966 R. TANNER 3,234,556

BROADBAND BICONICAL WIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAMSHAPING PORTION Filed Feb. 23, 1962 3 Sheets-Sheet l INVENTOR.

ROBERT L. TANNER ATTORNEY Feb. 8, 1966 R. L. TANNER 3,234,556

BROADBAND BICONICAL WIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAMSHAPING PORTION Filed Feb. 25, 1962 3 Sheets-Sheet 2 53 47 aw; 5 55 lL1| 4%? 1L 49 INVENTOR.

ROBERT L. TANNER ATTORNEY Feb. 8, 1966 L TANNER NA 3,234,556

R. BRO AND B NICA IRE- ID LENS ANTEN CO R SING ENTR BEA HAPING POR NFiled Feb. 23, 1962 A Sheets-Sheet 5 IOO INVENTOR ROBERT L. TANNER J um-316mm? ATTORNEY United States Patent 3,234,556 BROADBAND BICONICALWIRE-GRID LENS ANTENNA COMPRISING A CENTRAL BEAltl SHAPING PORTIONRobert L. Tanner, 4780 Alpine Road, Menlo Park, Calif. Filed Feb. 23,1962, Ser. No. 175,369 16 Claims. (Cl. 343-753) This invention relatesto lens-type antennas and more particularly to a broad band uniformgrid-wire antenna for operating :over bands lying within the frequencyrange extending from below 1 to above 1,000 megacycles per second.

Present day attennas for operating Within the above stated frequencyrange are of many types. In one very important portion of the range, theso-called HF (highfrequency) range, which embraces frequencies fromapproximately 3 mc. (megacycles per second) to 30 mc., and which is usedfor long distance radio communication, one type of antenna is therhombic antenna. The rhombic antenna consists of four current carryingconductors, each several wavelengths long, in the form of a diamond orrhombus. The structure is fed at one end by a transmission line andterminated at the other end in a resistance. The two sides of therhombus are, in effect, a continuation of the feeding transmission linein which the conductors diverge from the feed to the middle of therhombus and then converge again to the terminating resistance. Therhombic antenna transmits energy in the direction of the termination. Asa receiving antenna it receives from that direction.

Another Class of antennas is .the linear array. This antenna is composedof many elementary antennas usually resonant dipole antennasarrayedalong a line or in a plane. The spacing between the elements of thearray is usually about one-half wavelength. The purpose of arranging theelements in an array is that it permits the electromagnetic energy to beconcentrated in a relatively narrow beam. The beam may be in the line ofthe arrayin which case the array is known as an end fire array-or atright angles to itwhen it is called a broadside array.

Another type of array which is finding increasing use is the circular orWullenweber array, in which the elementary antennas are arranged alongthe periphery of a circle. In this antenna a beam is formed by feedingthe elements in a segment of the array equal to approximately half ofthe elements of the array. Each element is fed through a separateindividual phasing network. By means of these networks the currents inthe different elements are adjusted so that their radiations add to forma relatively sharp beam. The beam is in a direction which bisects theare formed by the excited elements. The useful feature of theWullenweber array is that the direction of the beam can be changed byswitching the feeds to a corresponding group of elements at a differentangular position in the array.

The antennas just described are examples of antenna types that haveproved useful in different applications. The rhombic antenna, forexample, has for many years been the most commonly used antenna in longdistance radio communication service. It has the advantages ofsimplicity, relatively low cost of construction and reasonably goodperformance. Linear array antennas have been frequently used for shortwave broadcasting applications for long distance communications and asradar 3,234,556 Patented Feb. 8, 1966 antennas. The Wullenweber array,because of its scanning capability, has been most frequently used as ahighresolution direction finding antenna.

Although the antennas described and other existing antennas have proveduseful, they all have certain inherent limitations. For example, therhombic antenna, although inexpensive to build it land costs areignored, makes very inefficient use of land area. A single large rhombicantenna for operation over a 4 to 10 mc. band might require a land areaof 10 acres or more. Another limitation is the useful operatingbandwidth. Rhombic antennas cannot be operated over frequency bandwidthgreater than 2 or 2 /2 to 1 without excessive deterioration in radiationpatterns. Thus to cover the band from 4 to 30 rnc. used in long distanceradio communications at least two separate antennas, with a total landrequirement of 15 to 20 arces, will be needed for each circuit. Thus ina communication station serving 20 different circuits the total landarea required for the installation of rhombic antennas could be as highas 400 acres.

A further limitation of rhombic antennas is the high level of side lobesand back lobes. Principal side lobes may be only 6 db below the mainlobe. When used as a receiving antenna the high side lobes of therhombic make it susceptible to interfering signals arriving at anglesdifferent from the principal direction. When it is used as transmittingantenna, the high side lobes cause energy to be radiated in other thanthe desired direction, giving rise to signals interfering with othercircuits, either nearby or in other parts of the world. Still anotherlimitation is that the beam of the rhombic cannot be scanned.

The linear array is also subject to certain limitations. The usefuloperating bandwidth is even narrower than that of the rhombic antenna.In addition, such antennas may be substantially more expensive toconstruct. Their beams can be steered over relatively small angles, byvarying the phases of the currents in the different elements of thearray, but this is accomplished only at the expense of substantialcomplication in the feed system, and a consequent increase in cost.

The Wullenweber array can be scanned over the entire 360 degree range ofazimuth. Because of the many separate phasing circuits and switchesrequired, however, the Wullenweber is very expensive to build. Powerhandling limitations in the phasing and switching networks tend to limitthe use of the Wullenweber to receiving applications. In addition,although the Wullenweber can be scanned to any azimuth angle, it islimited to operation at a single azimuth angle at any given time.

It is therefore a primary object of this invention to provide abroadband, low side-lobe and low back-lobe antenna for an operatingfrequency lying within the frequency range from below 1 mc. tosubstantially above 1,000 mc. The antenna of this invention also is tobe capable of operation over a frequency bandwidth greater than 10 to 1.

It is a further object of this invention to provide an antennaparticularly suitable for use in the HF and VHF frequency ranges, whichrelative to its performance, is small in physical size, inexpensive toconstruct and install, and light in weight.

It is a still further object of this invention to provide a broadbandantenna of the stationary type which is capable of scanning a beam,either by moving the feed to a different point on the perimeter of theantenna or by switching from one to another of many feeds spaced aroundthe perimeter.

It is another object of this invention to provide an antenna which iscapable of forming a plurality of beams in a plurality of directions bysimultaneously exciting a plurality of feeds positioned at a pluralityof points on the perimeter of the antenna.

It is still another object of this invention to provide a broadbandlens-type antenna which can, more simply and efiiciently than devicesheretofore known, shape the electromagnetic energy from simple feedantenna into a beam of any desired sharpness, allow substantialflexibility in the shape of the beam, scan a narrow beam through a largearc, and which has adjustable features so that changes of radiationpattern can be made in the field with relative simplicity.

Briefly, the lens-type antenna of this invention is constructed of apair of opposed wire mesh grids. The size of the individual meshes isdimensioned to be small compared with the shortest operating wavelengthto provide a propagation characteristic which is substantiallyindependent of the operating frequency and isotropic.

In one embodiment of this invention the equivalent dielectric constantof wire mesh grid structure is changed by changing the distance betweenopposite grids from a distance of separation which is small comparedwith the mesh size to a distance of separation which is large comparedwith the mesh size. By properly varying the distance of separationbetween different portions of the grids, and thereby the velocity ofpropagation, an antenna having circularly symmetric beam formingproperties can be constructed.

In another embodiment of this invention the equivalent dielectricconstant of the wire mesh structure is changed either by inductivelyloading the meshes or capacitively loading across the wire-grids. Thevelocity of propagation varies with the degree of inductive andcapacitive loading, and by judicious choice of loading a lens having thedesired characteristics is provided.

The antenna of this invention will be described in connection with anantenna system for converting a point source of HF energy from a simplefeed antenna into a cophasal wave front. Furthermore, the antenna ofthis invention is constructed to be circularly symmetric, the pointsource being located at a peripheral portion of the antenna while themain beam is provided by the antenna from the diametrically opposedperipheral portion. A hypothesized optical structure having a similarcircularly symmetric focusing property is known as a Luneburg lens. Toobtain this symmetric focusing property, the Luneburg lens is fashionedfrom a disc or sphere of dielectric whose permittivity variesparabolically from a value of 2.0 at the center to a value of 1.0 at theperiphery.

Other objects and a better understanding of the invention may be had byreference to the following description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of a wire-grid lens antenna constructed inaccordance with this invention in which a desired change in thepropagation characteristic is obtained by a change in the spacingbetween opposite wiregrids;

FIG. 2 is a sectional view taken along lines 2-2 of FIG. 1;

FIG. 3 is a schematic top view of a Luneburg-type lens for converting aperipherally located point source into diametrically opposed cophasalplane wave;

FIG. 4 is a sectional view similar to the view shown in FIG. 2 of afurther embodiment of the wire-grid lens antenna;

FIG. 5 is an enlarged detail view in perspective of a pie-shaped portionof the wire-grid lens antenna of FIG. 4;

FIG. 6 is an illustrative diagram of wave propagation parallel to one ofthe wires of a square Wit m 115?" ful in explaining the theory ofoperation of this invention;

FIG. 7 is an illustrative diagram of wave propagation along a diagonalof the wires of a square wire mesh useful in explaining the theory ofoperation of this invention;

FIG. 8 is a top View of a lens portion constructed of hexagon meshwire-grids;

FIG. 9 is a perspective view of a further embodiment of a wire-grid lensportion of the antenna of this invention in which the change in thepropagation characteristic is obtained by selective capacitive loadingacross opposing wire meshes;

FIG. 10 is a perspective view of a still further embodiment of awire-grid lens portion of the antenna of this invention in which thechange in the propagation characteristic is obtained by selectiveinductive loading of the meshes of the wire meshes; and

FIG. 11 is a top view of a lens portion of a further embodiment of thisinvention.

Referring now to the drawings, and particularly to FIGURES l and 2thereof, there is shown a wire-grid lens antenna 20 constructed inaccordance with this invention and comprising an upper wire-grid 21 anda lower wiregrid 22. Wire-grids 21 and 22 are circular and are suspendedin opposite and overlying relation from a plurality of non-conductiveperipheral support members 23 such as wooden, plastic or fiber glasspoles.

Antenna 20 actually comprises a center portion 24 which forms awire-grid lens for azimuthal beam shaping and a peripheral portion 25which forms a radiating structure for elevational beam shaping and formatching the impedance between lens 24 and the surrounding space. In theembodiment of the invention shown in FIGURES 1 and 2 radiating structure25 is formed as a biconical, or radially flared horn.

As a practical matter, upper wire-grid 21 may include an upper lenswire-grid fastened securely to an aluminum ring 31 which is light inweight and which may also be supported upon a plurality of circularlyspaced nonconductive poles 32 as best shown in FIG. 2. Smoothly joinedto ring 31 is the inner edge of an upper horn wiregrid 33 in the form ofan annulus which has its outer edge supported by peripheral poles 23.Similarly, lower wiregrid 22 may be made in two parts, a lower lenswire-grid 34 fastened to an aluminum ring 35 supported upon poles 32below ring 31 and a lower horn wire-grid 36 fastened along its inneredge to ring 35 and supported along its outer edge by peripheral poles23.

Even though the radiating structure 25 of antenna 20 is shown in theform of a biconical horn of wire mesh similar to that used for theconstruction of lens 24, other types of radiators or wave launchingdevices may be used. These would include horns utilizing a differenttype of wire-grid. Horns formed of solid conductive sheet may likewisebe utilized. For example, a wire-grid having a different mesh structurefrom that employed in forming lens 24 may be used as shown in FIG. 5.

As will be explained in greater detail hereinafter, lens wire-grids 30and 34 are formed of metallic wires and may have a variety of differentmesh structures. For example, the mesh structure may be square,triangular or hexagonal. However, even though diflerent mesh structuresmay be employed, the two wire-grids 30 and 34 forming a lens must besubstantially identical to one another and must be oriented with respectto one another in such a manner that corresponding sides of oppositewire meshes lie in a common plane which is vertical to both wire-grids,at least in those lens portions in which wiregrid separation is equal toor less than the mesh size. The reason therefore is that when gridseparation is small, corresponding sides of opposite meshes formtwo-wire transmission lines.

Lens 24 having been formed with proper alignment of correspondinglyopposed meshes, has been found to have certain very importantproperties. As long as the operating wavelength is large compared to themesh size, the transmission characteristic of the space between Wiregrids 30 and 34 is substantially independent of the operating frequencyand depends solely on the distance of their separation. It has beendetermined that for all practical purposes good independence is obtainedby selecting the mesh to be equal to or less than one-sixth of theshortest operating wavelength. Accordingly, the mesh size determines thehighest frequency of operation of the lens of this invention.

It has also been found that as long as the distance between oppositewire-grids 30 and 34 is small compared to the mesh size, say one-tenth,the velocity of propagation is substantially /2 of the velocity oflight. Also, as long as the distance between opposite wire-grids 30 and34 is large compared to the mesh size, say four times, the velocity ofpropagation is substantially equal to the velocity of light.

For the two-dimensional circularly symmetric wire-grid lens antenna,such as antenna 20, to provide a sufficiently narrow azimuthal beam, ithas been found desirable to make the diameter of grid-wire lens 24several times larger than the longest operating wavelength. For example,if the lens diameter is selected to be three wavelengths at the lowestoperating frequency, the half-power beam width at this frequency will beapproximately degrees. Accordingly, the diameter of the grid-wire lensdetermines the lowest frequency of operation of this invention.

By way of summary then, the bandwidth of the wiregrid lens antenna isdetermined at the high frequency end by the mesh size and at the lowfrequency end by the lens diameter.

An example, which will best show the physical dimension of an antennaconstructed in accordance with this invention, will now be given for thecase in which the desirable variation of the transmission characteristicis obtainable by varying the effective dielectric constant between 1 and2. Assume a desired frequency range from 4 to megacycles per second. Thedesired mesh size is about one-sixth of the shortest wavelength, orabout 1.7 meters. The diameter for good resolution is about three timesthe longest wavelength or 225 meters. Variation of grid spacing toobtain a change of efiective dielectric constant from 1 to 2 is from aminimum spacing of about one-tenth of the mesh size, or 17 centimetersto a maximum spacing four times the mesh size, or about 6.7 meters.

These properties of lens 24 may be utilized to construct an infinitevariety of lenses, each having a desirable transmission characteristicwhich varies in accordance with grid spacing. By way of example, aparticularly useful lens 36 is shown in FIG. 3 which is formed toconvert a point source 37 located at its periphery into a cophasal linefront 38 diametrically opposite thereto. The path of energy isdesignated by reference numeral 39. A lens having the above statedproperty is known as the microwave equivalent of the optical Luneburglens. As can be seen from the plurality of paths 39 depicted, thebending, and therefore the velocity of propagation increases withincreasing angular deviation from the diametrical bisector passingthrough source 37. The required relation of the transmissioncharacteristic of a Luneburg lens is given by the relation Where:

It is the equivalent index of refraction for a wave propagating betweenthe wire grids;

a is the diameter of the wire-grid lens; and

r is the distance from the center of the wire-grid lens.

For a Luneburg type wire-grid lens constructed in accordance with thisinvention which will operate over a frequency range of 4 to 30 me. usinga 5 ft. square mesh wire grid of No. 8 wire (Radius-.064 inch) andhaving a diameter a of 600 ft. the following table provides thedistances of grid separation s as a function of distance from the lenscenter r.

r, feet s," inches Feet.

Generally speaking, to design a wire-grid lens in accordance with thisinvention, it is only necessary to derive an expression for thetransmission characteristic in terms of distance of separation of thewire-grids and the coordinates of the lens and thereafter determine sfor every point of the lens. The equivalent index of refraction is afunction of size and shape of the mesh, the diameter of the wire and thespacing between wire-grids. A formula for the index of refraction of asquare mesh is given in Properties of a Pair of Wire-Grids for Use inLens-Type HF Antennas by Andreasen and Tanner, 1961 Western ElectronicShow and Convention, paper No. l/ 3.

Referring now to FIGS. 4 and 5, there is shown a further embodiment ofthe wire-grid lens antenna in which the lower wire-grid is planar and inwhich a sectional construction is utilized, each section beingpie-shaped. An tenna 45 comprises a plurality of pie-shaped sections 45,each one of which includes a planarly suspended pieshaped lowerwire-grid 46 and a curvedly suspended pieshaped upper wire-grid 47. Thecenter of antenna 44 is formed by a non-conductive support pole 48 towhich the pointed end of the individual pie-shaped wire-grids areattached. Additional non-conductive support poles 49 are provided toassure a good durable suspension without undue sag.

Wire-grids 4-6 and 47 are of triangular mesh and are spaced in such amanner that the wires are substantially parallel to form individualtransmission lines. The peripheral edge of each pie-shaped wire-grid 47may be affixed to an aluminum ring 50 which is supported onnon-conductive support poles 49 and which provides a convenient edgesupport for a flared horn radiating structure 51.

Radiating structure 51 likewise includes a plurality of a lower and anupper annular wire-grid sectors 52 and 53, but instead of having atriangular mesh structure they are formed of square mesh. The outerperipheral edge of wire-grids sectors 53 are supported by nonconductivepoles 54 which may be anchored by means of guy wires 55. For properfunctioning, the guy wires must be broken at intervals by means ofinsulators. The inner peripheral edge of wire-grid sectors 53 arefastened to ring 50. Wire-grid sectors 52 are shown to form planarextension of lens wire-grids 46 to allow antenna 44 to be as close tothe ground as possible and to provide a slightly upwardly directed beam.Of course radiating structure 51 may be formed to have both its upperand its lower annular wall flared at any angle to provide the desiredelevational beam angle and beam pattern.

To explain the operation of the wire-grid lens of this invention,reference is made to FIGURES 6 and 7 which both show a portion of asquare-mesh wire-grid lens. FIGURE 6 shows a wire-grid lens portion 60,in which the upper wire grid is spatially positioned exactly over thelower wire grid of the pair of grids forming the lens. The lower grid isconsequently hidden from view by the upper wire grid. Representativewires forming the wire grids are shown as wires 61 and 62. FIGURE 7similarly shows a wire-grid lens portion 63 in which the upper wire gridis spatially positioned so as to hide the lower wire grid from view.Representative wires of the visible wire grid are 64 and 65.

Consider now a circumstance in which a wave is traveling toward theright through lens portion 63. The Wave advances along a broad front andtherefore may be considered to travel along many parallel corridors,each of which will have identical currents and fields. A representativecorridor is shown in FIGURE 7 contained between lines 74 and 75.Consider first the case in which the upper and lower grids are spaced adistance which is small compared to the distance between nodes of themesh. For this case, opposing wires in the upper and lower gridsconstitute, in effect, the two wires of a twowire transmission line, theplane of which is perpendicular to the plane of the grids. That portionof the wave which travels down the corridor defined by lines 74 and 75includes currents 72 and 73. Because the grids are so close togetherthat the opposing wire members have the properties of two-wiretransmission lines, the wave energy is constrained to follow thedirection of the wires. In advancing between successive mesh nodes alongthe center of the corridor between 74 and 75, the wave advances adistance equal to the diagonal of the mesh square. Because it wasconstrained to follow the Wires, however, the actual distance travelledby the Wave is equal to two times the mesh side. The distance actuallytraveled is therefore greater than the distance of effective advance bythe ratio VT, which is the ratio of twice the side of a square to itsdiagonal. As a consequence, the wave has been effectively slowed by thefactor equal to /2.

Consider now the case illustrated in FIGURE 6 where the wave travelsparallel to wire 62. Again, because it advances on a broad front it canbe considered as traveling down many parallel corridors. In FIGURE 6such a corridor is defined by lines 66 and 67. Because of the symmetryof currents in adjacent corridors, no current flows across these lines,which is equivalent to saying, in considering the portion of the wavetraveling down this corridor, that lines 66 and 67 could be replaced bya perfect open circuit, and the remainder of the grid structure oneither side of the corridor removed.

Because of the fact that the grids are spaced so closely together thatopposing wires in the upper and lower grids constitute two-wiretransmission lines, the structure contained in the corridor defined by66 and 67 is now equivalent to a central two-wire transmission linecarrying current 70 loaded at intervals equal to the mesh side length bythe short sections or stubs of open-circuited two-wire transmission lineinto which the cross wires degenerate. The impedance presented by thesmall open-circuited stubs is capactive, and their effect is to apply tothe transmission line formed by the central wires of the corridor aperiodic capactive loading which in effect doubles the capacitance perunit length of that line, but leaves its inductance unchanged.

It is well known that the wave velocity on an open two-wire transmissionline is equal to the velocity of light in free space and is given by theformula where C is the capacitance per unit length of the transmissionline and L is the inductance per unit length. Because the effect of thecross wires of the grid is to apply a capactive loading which doublesthe effective capacitance per unit length of the line, the velocity ofthe wave in the grid is seen to be It is therefore evident that a wavetraveling parallel to one set of grid wires is slowed by a factor equalto /2, just as was the case with a wave traveling in a directiondiagonal to the grid wires. The same slowing factor holds true for awave traveling in any direction in the space between the grids providedthat the mesh size of the grids is small compared with the wavelengthand the spacing between the grids is small compared with the mesh size.Therefore, the wire grid pair for this condition is also substantiallyisotropic.

Now consider the situation if the spacing between the grids is increasedto a distance that is large compared with the mesh size. It is wellknown that under most circumstances a Wire grid having a mesh size smallcompared with the wavelength behaves electrically the same as if it werea solid metal plate. This is the condition that prevails when the gridsare separated a distance which is large relative to the mesh size. It isalso well known that a wave propagating between parallel metal plates inthe so-called TEM mode (transverse electro-magnetic), which has itselectric field vector polarized perpendicular to the plane of the metalplates, travels at free space velocity.

Thus it is evident that when the grids are spaced a distance largecompared with the mesh size so that they simulate metal plates aquasi-TEM wave propagating in the space between them travels at freespace velocity. When the spacing between the grids is reduced, the Waveis slowed. Maximum slowing is achieved when the gridto-grid spacing ismade small compared with the mesh size, in which case the wave is slowedby a factor equal to /2. From the foregoing discussion it is alsoevident that the wave propagating between the grids has its majorcomponent of electric field polarized perpendicular to the plane of thegrids. All wave velocities between free space velocity and l/ /2 timesfree space velocity can be obtained by an appropriate spacing of thegrids relative to the mesh size. The relation between wave velocity andgrid parameters is given in the previouslyreference paper by Andreasenand Tanner.

Although the preceding discussion is carried out in terms of square meshgrids, other mesh shapes also work. Specifically, grids composed ofmeshes having the form of equalateral triangles and regular hexagons areboth suitable. An analysis similar to that carried out for square meshgrids shows that in the case of grids spaced a distance small comparedwith the mesh size,

a slowing factor of /2 is obtained with both triangular mesh grids andhexagonal mesh grids.

As already mentioned, the square-mesh and the triangular-mesh wire-gridsare not the only grid structures useful in practicing this invention. Infact, since both the triangular-mesh and hexagonal-mesh have more planesof symmetry, they are less anisotropic. Also, the hexagonal mesh hasbeen found to show the least frequency dependence.

FIG. 8 shows a hexagonal-mesh wire-grid which has been found useful inpracticing the invention.

FIGS. 9, l0 and 11 show three further embodiments of wire-grid lenses.In the wire-grid lens of FIG. 9 the Wave velocity of a wave propagatingbetween an upper wire-grid 91 and a lower wire-grid 92 is slowed down byselective capacitive loading. Generally, Wiregrids 91 and 92 may besuspended in overlying relation and a number of capacitive loads such as93 connected across junction points 94. The advantage of a capacitiveloading is that the velocity may be decreased below /Z of the velocityof light, the lowest velocity obtainable with unloaded Wire-grid lenses.In other words, capacitive loads provide a means for increasing therange through which the velocity may be changed.

FIG. 10 shows a wire-grid lens comprising an upper wire-grid 101overlying a lower wire-gird 102. The wave velocity is decreased byinserting inductive loading into the wires between junction points suchas inductive loads 103. Just as before, the velocity can be decreased bya value below /2 of that of the free space velocity by utilizing lumpedinductors 103, thereby overcoming the range limitation of changing thedistance of separation of overlying spaced wire-grids.

In certain instances, a wire-grid lens antenna may be constructedincorporating a change of wire-grid spacing in combination withcapacitive and/ or inductive loading. In this manner the wave velocitymay be changed from that of free space velocity to /2 of free spacevelocity by utilizing the rather convenient method of varying theseparation between wire-grids and in those places where a furtherdecrease of velocity is desired, capacitive or inductive loading isadded.

FIG. 11 shows a wire-grid lens 110 comprising a pair of overlyingwire-grids, the upper one being designated by reference character 111and the lower one being hidden from view by grid 111. The wave velocityis decreased in lens 116* by physically lengthening the path from onemesh junction point, say 112, to the adjacent junction point, say 113.This lengthening of path is accomplished by including a zig-zag portion114 in the wire forming the side of a mesh.

By way of summary, the wave velocity in a wire-grid lens is equal to thevelocity of light when the grid spacing is large in comparison with themesh size. The wave velocity can be decreased by decreasing the spacingbetween wire-grids. In this manner the slowest wave velocity obtainableis /2 the velocity of light. The wave velocity can also be decreased byinductive and capacitive loading which produces a phase shift. Lastly,instead of changing the phase by electric impedance means, the velocitymay also be decreased by increasing the physical path length of thetransmission line as shown in FIG. 11.

Feeding of the grid-wire lens antenna may be accomplished byconventional means such as a feed horn mounted for rotation along theperipheral portion of the lens antenna. In case of directional antennaswhich radiate along different azimuthal directions, a single lensstructure with fixed feed horns facing the azimuthal direction ofradiation provide a very useful structure.

There has been described a wire-grid lens antenna utilizing a pair ofoverlying wire-grids to form the lens portion and a radiating structuremounted to the end of the lens portion. The wave velocity may be changedby (l) changing in spacing between grids or (2) capacitive loading ofthe grid or (3) inductive loading of the grid or (4) increasing thetransmission line geometrically between mesh junctions. The antenna ofthis invention remains relatively insensitive to frequency as long asthe mesh size is small compared with the shortest operating wavelength.

What is claimed is:

1. A wire-grid lens antenna comprising a pair of spaced, overlying,conductive wire-grids having substantially the same mesh structure, themesh openings of said wire-grids having a size which is small incomparison with the shortest operating wavelength and being arranged insubstantial spaced alignment with one another, said spaced Wire gridsdefining therebetween a Wave propagating region for a wave polarizedwith its electric field perpendicular to the plane of the Wire grid,means for supporting said overlying wire-grids at each point of saidantenna with a spacing varying from a distance which is large incomparison with the mesh opening size to a distance which is small incomparison with the mesh opening size to provide a selected variation ofthe wave propagation velocity from that substantially equal to thevelocity of light to that substantially equal to 1/2 the velocity oflight.

2. A circularly symmetric wire-grid lens antenna for converting aperipherally located point source of radiation to a cophasally extendedsource directing a beam along the diametrical bisector of said antennawhich passes through said point source, said antenna comprising a pairof circular, spaced, overlying, conductive wire-grids arranged to form asurface of revolution about the axis of said antenna, the said spacedwire grids defining therebetween a wave propagating region for a wavepolarized with its electric field perpendicular to the plane of the wiregrid, mesh opening size of said wire-grids being less than one-fourth ofthe shortest operating wavelength and the spacing of said wire-gridsincreasing in a radially outwardly going direction from a distance whichis small compared with the mesh opening size to a distance which islarge compared with the mesh opening size, and non-conductive means forsupporting and spacing said wire-grids.

3. A circularly symmetric wire-grid lens antenna for converting aperipherally located point source of radiation to a cophasally extendedsource directing a beam along the diametrical bisector of said antennawhich passes through said point source, said antenna comprising a pairof circular, spaced, overlying conductive wiregrids arranged to form asurface of revolution about the axis of said antenna, said spaced Wiregrids defining therebetween a wave propagating region for a wavepolarized with its electric field perpendicular to the plane of the wiregrid, the mesh opening size of said wire-grids being less thanone-fourth of the shortest operating wavelength, the diameter of saidwire-grids being greater than twice the longest operating wavelength,and non-conductive means supporting said wire-grids with a spacing whichincreases in a radially outwardly going direction from a distance whichis small compared with the mesh opening size to a distance which islarge compared with the mesh opening size.

4. A circularly symmetric wire-grid lens antenna for converting aperipherally located point source of radiation to a cophasally extendedsource directing a beam along the diametrical bisector of said antennawhich passes through said point source, said antenna comprising a pairof circular, spaced, overlying conductive wire-grids arranged to form asurface of revolution about the axis of said antenna, said spaced wiregrids defining therebetween a wave propagating region for a wavepolarized with its electric field perpendicular to the plane of the wiregrid, the mesh opening size of said Wire-grids being less thanone-fourth of the shortest operating wavelength, the diameter of saidwire-grids being greater than twice the largest operating wavelength andthe spacing of said wiregrids increasing in a radially outwardly goingdirection from a distance which is less than one-half the mesh openingsize to a distance which is greater than twice the mesh opening size, aradiating structure afiixed to the peripheral edge of said pair ofwire-grids, and non-conductive means for supporting and spacing saidWire-grids.

5. A wire-grid lens antenna in accordance with claim 4 in which saidvariations in wire-grid spacing is such that the effective dielectricconstant varies parabolically.

6. A wire-grid lens antenna comprising a central radiation beam shapingstructure with a beam radiating structure on the periphery, said centralbeam shaping structure including a pair of conductive, spaced, overlyingwire-grids whose mesh opening size is less than onefourth of theshortest operating wavelength, said spaced wire grids definingtherebetween a Wave propagating region for a wave polarized with itselectric field perpendicular to the plane of the wire grid, the spacingbetween opposite points on said pair of wire-grids being selected toprovide a desired predetermined relative effective permittivity.

7. A wire-grid lens antenna comprising a central radiation beam shapingstructure with a beam radiating structure on the periphery, said centralbeam shaping structure including a pair of conductive, spaced, overlyingwiregrids each having a mesh opening size of less than onefourth of theshortest operating Wavelength and having a maximum wire-grid separationof less than five times the mesh size, said spaced wire grids definingtherebetween a wave propagating region for a Wave polarized with itselectric field perpendicular to the plane of the wire grid, and lumpedimpedance means associated with certain meshes for decreasing therelative efiective permittivity between said certain meshes.

8. A wire-grid lens in accordance with claim 7 in which said lumpedimpedance are inductors connected in series with the wires forming thesides of said certain meshes.

9. A wire-grid lens in accordance with claim 7 in which said lumpedimpedance are capacitors connected between opposite junctions formingthe corners of said certain meshes.

10. A Wire-grid lens antenna comprising a central radiation beam shapingstructure with a beam radiating structure on the periphery, said centralbeam shaping structure including a pair of conductive, spaced, overlyingwiregrids having a mesh opening size of less than one-fourth of theshortest operating wavelength, said spaced wire grids definingtherebetween a Wave propagating region for a wave polarized withitselectric field perpendicular to the plane of the wire grid, certainpairs of overlying meshes being formed of Wires which include a zig-zagportion for increasing the mesh size, the increase in electrical pathlength being selected to provide a predetermined decrease in therelative elfective permittivity in the region of said certain meshes.

11. A wire-grid lens antenna comprising a substantially circular centralbeam shaping portion surrounded by a beam radiating portion, saidcentral beam shaping portion comprising a pair of conductive, spaced,overlying wire-grids arranged to form a surface of revolution about theaxis of said antenna, said spaced Wire grids defining therebetween awave propagating region for a wave polarized with its electric fieldperpendicular to the plane of the wire grid, the mesh size of saidwire-grids being less than one-fourth of the shortest operatingwavelength, said beam radiating portion extending from said pair ofcircular wire-grids in the form of a biconical horn.

12. A wire-grid lens antenna comprising a central beam shaping structuresurrounded by a beam radiating structure, said central beam shapingstructure including a pair of spaced, overlying conductive wire-grids ofthe same mesh structure, said spaced wire grids defining therebetween awave propagating region for a wave polarized with its electric fieldperpendicular to the plane of the wire grid, the mesh openings of saidwire-grids having a size which is small in comparison with the shortestoperating wavelength and being arranged in substantial spaced alignmentwith one another, the spacing between overlying 113. In a wire-grid lensantenna the improvement in the A lens comprising a pair of spaced,overlying, conductive wire-grid meshes having a mesh opening size whichis smaller than one-fourth of the shortest operating wavelength, andmeans for supporting said grid meshes relative to one another to flareoutwardly in all directions from their center at a rate increasing withthe radial distance from their center whereby a plane Wave applied toone edge of said pair of grid meshes with a polarization such that itselectric field is perpendicular to the plane of said grid meshes isfocused substantially at a point at the opposite edge.

14. In a wire-grid lens antenna as recited in claim 13 wherein thegeometrical configuration of the individual meshes in said wire-grid arein the shape of an equilateral triangle.

15. In a wire-grid lens antenna as recited in claim 13 wherein thegeometrical configuration of the individual meshes in said Wire-grid arein the shape of a square.

16. In a wire-grid lens antenna as recited in claim 13 wherein thegeometrical configuration of the individual meshes in said wire-grid arein the shape of a hexagon.

References Eited by the Examiner UNITED STATES PATENTS 2,485,138 10/1949Carter 343-915 2,511,916 6/ 1950 Hollingsworth 333- 2,576,181 11/1951Iams 343754 2,576,182 11/1951 Wilkinson 343754 2,596,251 5/1952 Kock343-909 2,720,588 10/1955 Jones 343754 2,756,424 7/1956 Lewis 343-9092,884,629 4/1959 Mason 343780 3,047,860 7/ 1962 Swallow 343897 3,116,48512/1963 Garson 343754 FOREIGN PATENTS 402,834 12/ 1933 Great Britain.

OTHER REFERENCES Silver: Microwave Antenna Theory MIT Rad. Lab. Series,vol. 12, page 449 relied upon.

HERMAN KARL SAALBACH, Primary Examiner.

1. A WIRE-GRID LENS ANTENNA COMPRISING A PAIR OF SPACED, OVERLYING,CONDUCTIVE WIRE-GRIDS HAVING SUBSTANTIALLY THE SAME MESH STRUCTURE, THEMESH OPENINGS OF SAID WIRE-GRIDS HAVING A SIZE WHICH IS SMALL INCOMPARISON WITH THE SHORTEST OPERATING WAVELENGTH AND BEING ARRANGED INSUBSTANTIAL SPACED ALIGNMENT WITH ONE ANOTHER, SAID SPACED WIRE GRIDSDEFINING THEREBETWEEN A WAVE PROPAGATING REGION FOR A WAVE POLARIZEDWITH ITS ELECTRIC FIELD PERPENDICULAR TO THE PLANE OF THE WIRE GRID,MEANS FOR SUPPORTING SAID OVERLYING WIRE-GRIDS AT EACH POINT OF SAIDANTENNA